1. Field of the Invention
The present invention generally relates to a ladder type piezoelectric filter used in filter circuits of radio communication apparatuses such as portable mobile radio transceivers, mobile radiotelephones or the like and piezoelectric resonator therefor. More particularly, the invention relates to a high-frequency ladder type piezoelectric filter used in high-frequency ranges not lower than 1 MHz.
2. Description of the Background Art
Conventionally, most ladder type piezoelectric filters of this general type are designed to be operable at the response frequency of 455 KHz. This type of piezoelectric filter is so arranged that both the piezoelectric resonators made in large thicknesses and small capacities with the resonant frequency of 455 KHz, and the piezoelectric resonators made in small thicknesses and large capacities with the antiresonant frequency of 455 KHz are combined with each other in the series and parallel forms, respectively, thereby forming the ladder type filter circuit.
On the other hand, recently, ladder type piezoelectric filters operable in the high frequency ranges such as 1.7 MHz or 2 MHz are required as filter devices employed in radio communication apparatus for mobile telephones. However, when the above-described conventional ladder type piezoelectric filters are redesigned so as to satisfy such requirements, although the dimension of the piezoelectric resonator operated at the frequency of 455 KHz is originally 5 mm, this dimension should be made smaller than 2 mm. Accordingly, the ladder type piezoelectric filter configured by such small sized resonators could not be realized for practical reasons.
With respect to the conventional disk-shaped piezoelectric resonator, or rectangular plate-like piezoelectric resonator having the above-described dimension of 5 mm, which vibrates along the circumferential direction, a second harmonic mode SM and a third harmonic mode TM appear together with the fundamental mode FM having the large resonance impedance around 500 KHz as represented in FIG. 12 and FIG. 13. The above-described resonators operated at 455 KHz utilize the fundamental mode FM. Even in these conventional resonators, the second and third harmonic wave modes SM and TM appear at the frequencies not lower than 1 MHz. As a result, these harmonic modes may be utilized in such a ladder type piezoelectric filter. However, since the resonance impedances of these harmonic modes are relatively low as compared with that of the fundamental mode, these harmonic modes could not be practically utilized in the piezoelectric filter.
In order to solve the above problems, the following ladder type piezoelectric filter capable of being used in high frequency ranges has been proposed. The proposed ladder type piezoelectric filter includes a series resonator and a parallel resonator. In the series resonator, a center electrode and an outer electrode are provided in a concentric form on one surface of a piezoelectric ceramics plate, and a common electrode is provided on the other surface of the plate, in which a center electrode region and outer electrode region are mutually polarized along the same direction. On the other hand, in the parallel resonator, a center electrode and an outer electrode are provided in a concentric form on one surface of a piezoelectric ceramic plate, and a common electrode is provided on the other surface of the plate, in which a center electrode region and an outer electrode region are mutually polarized along opposite directions. Then, the ladder type circuit is arranged by a plurality of thus configured piezoelectric resonators representing resonant frequencies and antiresonant frequencies higher than 1 MHz by using the second harmonic waves.
In the thus proposed ladder type piezoelectric filter, the center electrode of the series resonator and the outer electrode thereof located on the same surface of the piezoelectric ceramic plate are successively connected to the input/output line. As a consequence, with respect to such a ladder type piezoelectric filter in that the respective resonators are housed within the case in a stacked manner, desirable electric connections should be conducted, while these center electrodes and outer electrodes should be electrically insulated. Therefore, the structure of the terminal plates would become complex, and further it is rather difficult to stably maintain the electric conducting paths. Accordingly, there are problems that the shortcircuits would likely occur and thus the final products would fail.